1. Field of the Invention
The present invention relates to a motor drive circuit for driving a synchronous motor.
2. Description of the Related Art
There are various kinds of synchronous motors. For example, known synchronous motors are a surface permanent magnet synchronous motor (SPMSM), a permanent magnet (PM) type stepping motor, a variable reluctance (VR) type stepping motor, a hybrid (HB) type stepping motor, a brushless DC motor (BLDCM) and the like. As methods for detecting the rotational position of a synchronous motor, there are a method using a sensor and a method without using the sensor.
As the method without using the sensor, there is a method using the induction power induced by a stator and a rotator (hereinafter referred to as “speed electromotive force” as appropriate). As the method using the speed electromotive force, there are (1) a method where a vector is calculated from the voltage/current of a motor stator and a motor model formula so as to estimate the position and (2) a method where a drive line of the motor is set in a high impedance state for a specific period of time and the speed electromotive voltage is directly detected. In the method (1), the position of the rotor can be estimated over the entire period of the drive periods. In the method (2), the position of the rotor can be estimated at the zero-cross timing of a speed electromotive voltage component detected.
In the method (1), the circuit scale increases and thereby the cost is high. In the method (2), conducting current to a motor must be stopped for a period of time during which the speed electromotive voltage is detected. Hence, the continuity of current is lost. Also, the sampling point to estimate the position of the rotor is restricted to the zero-cross timing of a speed electromotive voltage. In particular, in the case of 2-phase motor, the state of the rotor can be detected for every ¼ electrical angle cycle only, even if the nonconductive periods of each phase are summed up in this method (2).